1. Field of the Invention
The present invention relates to a power supply apparatus, and more particularly to a power supply apparatus that can detect the voltages of the individual battery modules constituting a vehicle-driving battery for driving a motor for making a vehicle run.
2. Description of Related Art
A vehicle-oriented power supply apparatus drives a motor for making the vehicle run. The motor is required to yield an output sufficiently high to make the vehicle run. To make the motor yield a high output, the vehicle-oriented power supply apparatus is typically designed to yield an extremely high output voltage of 200 V to 400V. To produce such a high output voltage, the power supply apparatus incorporates a vehicle-driving battery having a large number of battery modules connected in series. Moreover, each battery module is composed of a plurality of secondary cells connected in series. Used as the secondary cells are, for example, nickel hydride cells or lithium ion secondary cells.
In a vehicle-driving battery composed of a large number of battery modules, an equal charge/discharge current flows through all the battery modules. However, not all the battery modules have identical electrical characteristics. For example, when any of the battery modules deteriorates to have a lower full-charge capacity, this battery module becomes prone to be overcharged and overdischarged. Overcharging and overdischarging make batteries deteriorate. Thus, if any of the battery modules deteriorates, this battery module tends to deteriorate faster. To prevent this inconvenience, in a vehicle-oriented power supply apparatus, the voltages of all the battery modules are detected. The voltages of the battery modules are important indicators of their states. Based on these indicators, the charging and discharging of the vehicle-driving battery are controlled while protection is provided for the individual battery modules.
In a power supply apparatus that is composed of a large number of battery modules connected in series and that detects the voltages of all the battery modules, it is difficult to always accurately detect the voltages of the individual battery modules. This is because detecting the voltages of the individual battery modules requires the use of a large number of long leads, and because connecting those leads requires the use of a large number of connectors. Moreover, a vehicle-oriented power supply apparatus is used in an environment involving extremely harsh extrinsic conditions in terms of, for example, temperature and humidity, and this too makes the detection of the voltages of a large number of battery modules difficult. Furthermore, a vehicle is used in a harsh environment for a long period of time, and this makes the contacts of connectors prone to get soiled and oxidized. This makes accurate detection of the voltages of the battery modules further difficult.
Consider, for example, a vehicle-oriented power supply apparatus composed of 50 battery modules connected in series. If the charging or discharging of the vehicle-driving battery is restricted or inhibited just because the voltage of one battery module cannot be detected normally, the normal running of the vehicle will be severely limited. To avoid this inconvenience, there have conventionally been developed techniques that leave the running of a vehicle unlimited when simply the normal detection of the voltage of a battery module is found impossible and not that the actually detected voltage of a battery module is found abnormal. One example of such a technique is disclosed in Japanese Patent Application Laid-Open No. H11-176480 (hereinafter Patent Publication 1).
The power supply apparatus disclosed in Patent Publication 1 mentioned above detects the voltages of individual battery modules and the overall voltage of a vehicle-driving battery. Then, based on the voltages of the individual battery modules, information on their respective states is fed to an electronic control unit, and, based on the overall voltage, information on the state of the entire stack of batteries is fed to the electronic control unit. For each of the overall voltage and the individual battery module voltages, whether or not it is detected normally is checked. If the overall voltage is found to be detected erroneously, the voltages of the battery modules are added together so as to be used as the corrected overall voltage. If the voltage of a given battery module is found to be detected erroneously, either the mean value of the voltages of the battery modules as calculated from the overall voltage or the value calculated by subtracting the sum of the voltages of the normal battery modules from the overall voltage is used as the corrected voltage of the given battery module.
Disadvantageously, however, this method requires the detection of both the overall voltage and the voltages of the individual battery modules, and thus requires complicated arithmetic processing.